Tweak TestHarness API and docs #2429

Job	Run time
cargo check (aarch64-android)	24s
cargo check (msrv) (windows-latest)	2m 29s
cargo check (msrv) (wasm32)	19s
cargo check (msrv) (macos-latest)	35s
cargo clippy (wasm32)	1m 57s
cargo check (msrv) (ubuntu-latest)	37s
cargo doc	41s
cargo clippy (windows-latest)	4m 58s
cargo clippy (macos-latest)	2m 24s
Prime LFS Cache	4s
cargo test (wasm32)	18s
cargo clippy (ubuntu-latest)	3m 32s
formatting	33s
typos	7s
cargo test (windows-latest)	2m 36s
cargo test (macos-latest)	1m 18s
cargo test (ubuntu-latest)	3m 9s
	26m 1s

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